Evaluation of Retention of two Different Cast Post-Core Systems and Fracture Resistance of the Restored Teeth.

STATEMENT OF THE PROBLEM
The survival of pulpless teeth restored with different post and core systems is still a controversial issue.


PURPOSE
This study compared the retention of two different post and core systems and also the fracture resistance of teeth restored with these systems.


MATERIAL AND METHOD
Eighty endodontically treated maxillary central incisors were sectioned perpendicular to the long axis at a point 2mm incisal to the cemento-enamel junction (CEJ) and then the root canals were obturated. The restored teeth were randomly divided into two equal groups of 40. One group was restored with Nickel-Chromium (Ni-Cr) post and core system and the other group with Non-Precious Gold alloy (NPG) system. For evaluation of fracture resistance of the restored teeth, the specimens (n=20 per each group) were mounted in acrylic resin blocks and a layer of polyvinyl siloxane was applied to cover the roots. Loads were applied at an angle of 45 degrees to the long axis of the teeth and measured with a universal testing machine. The axial retention values of the studied groups (no=20) were measured on an Instron testing machine at a crosshead speed of 0.5 mm/min. Statistical analyses were performed using SPSS version 19.00 and student's t-test (α=0.05).


RESULTS
Although retention failure load for Ni-Cr system was lower than NPG system, there was no significant difference between the two systems (p= 0.7). However, fracture resistance of the teeth restored with Ni-Cr post and core system was significantly higher than NPG group (p= 0.000).


CONCLUSION
There was no significant difference between the retention of the studied post and core systems. Although significantly higher fracture thresholds were recorded for Ni-Cr post and core group, the failure loads of both systems may rarely occur clinically.


Introduction
Dental practitioners are often faced with the task of restoring endodontically treated teeth. Root canal treatment is usually the consequence of caries followed by pulpal infection or traumatic damage to a tooth. Trauma and caries are mostly associated with an extensive loss of tooth structure. [1] The pulpless tooth has already lost substantial coronal tooth structure from the access preparation for the endodontic treatment [2] which necessitates restoration of the tooth with a complete crown for esthetic and functional rehabilitation. When a large portion of the [10][11]17] Currently, the material of choice for custom-made cast-metal dowel-core system is Ni-Cr alloy. Due to high rigidity of Ni-Cr posts, less reduction of tooth structure is needed so that the maximum retention and fracture resistance of the post will be provided. [7,17] In the study conducted by Hayashi et al., [18] the teeth restored with Ni-Cr cast posts had significantly higher fracture resistance than other groups and had the lowest risk of vertical root fracture. Also, Ni-Cr alloys create a layer of chromium oxide that resists tarnish.
These alloys have also some disadvantages, for example, most of them have a breaking point over half of the root length due to the high post stiffness. If these teeth become fractured, they are unrepairable. [13][14][15][16] The other disadvantages are difficulty in finishing and polishing processes, doubt in biocompatibility due to presence of nickel and subsequent allergic reactions, [13] and absence of physical characteristics similar to dentin.
However, in spite of these disadvantages, it still has the highest usage in fabricating post-cores. [18] Controversial results have been reported from in vitro researches in regard to various post and core system. For instance, Dakshinamurthy et al. [19] indicated that the highest fracture resistance was recorded with Ni-Cr cast post and core in comparison with prefabricated titanium post and core. Maccari et al. [20] showed that the teeth restored with cast posts had fracture strength twice as high as the teeth restored with resin post. Assif et al. [21] made a comparison between the fracture resistance of the teeth restored with cast postcores and those restored with various types of post designs and did not detect any significant differences.
McDonald et al. [22] compared the fracture resistance of the teeth restored with a cast post, teeth restored with a carbon-fiber post, and intact root-treated teeth (controls); they found no significant differences among them. [23] De Castro Albuquerque et al., [24] Amussen et al. [25] and Lanza et al. [26] reported that glass fiber and cast post and cores had similar fracture resistance regardless of the type of the tooth. Trabert et al. [27] concluded that conservation of tooth structure enhanced resistance to fracture regardless of the design of the post. Hoag and Dwyer [28] determined that a complete cast crown on extracted molars was more important than the type of post and core technique for preventing fracture. However, Kern et al. [29] noted no statistically significant improvement in sheer strength of postreinforced extracted molars when the crowns were implemented. [30]  software, the teeth were randomly allocated into two groups (n=40). Group A was restored with dowels and cores fabricated using Ni-Cr and group B was restored with dowel and cores fabricated by using NPG alloy.
The composition, characteristics and manufacturer of each alloy is presented in Table 1.
The teeth were immersed in 5% sodium hypochlorite for 15 minutes in order to remove the organic materials from the root surfaces. Any remaining tissue was carefully cleaned by using a periodontal curette and then stored in distilled water.
The crowns were cut into horizontal sections, perpendicular to the long axis at a line 2mm incisal to the most coronal point of the proximal cementoenamel junction (CEJ), using diamond discs mounted on a lathe-cut machine under continuous water coolant.

Mounting the Teeth in Acrylic Blocks
The specimens were individually mounted vertically in self-cure acrylic resin in the root block former The roots were prepared to have 2mm ferruled collar with 5mm diameter, 6 convergences, and a 1mm shoulder finish line.

Root canal preparation and obturation
After preparing the access cavity by use of a high speed air motor (NSK; Tochigi, Japan), the working length The casts were poured with type IV die stone (Whip mix; Dortmond, Germany). Post and core models were waxed up and burned out identically into two groups.
Group A was injected with Ni-Cr and group B with NPG. In specimens for evaluating retention, a1mm diameter hole width was prepared in the core at the stage of wax up which was duplicated in the final core. The length of the core in all samples was accurately measured and made uniform at 4mm.

Dowel cementation
The dentin walls of the dowel space were etched with

Results
Fracture resistance of the restored teeth and retention of posts were measured using an Instron testing machine.
Data were evaluated statistically using Independent ttest. The mean and standard deviation values for frac-ture resistance of restored teeth have been summarized in Table 2 and Figure 1; values for post retention are shown in Table 3 and Figure 2. Although the mean retention of Ni-Cr system (101.01 N) was lower than NPG (117.02N), statistical analysis revealed no significant difference in regard to retention of the studied dowel-core systems (p= 0.70).    None of the specimens was broken down from core portion or either core-post interface.

Discussion
The results of the present study showed that there was a statistically significant difference between the fractures resistances of the teeth restored with Ni-Cr and those restored with NGP alloys; however, the retention of the two groups was relatively the same.
In this invitro study, the teeth extracted were care- buildup if the margins encircle a sound dentin collar. [7,[35][36][37] In this study, the test loads were applied directly on the cores, not to artificial crowns. If complete crowns with 2-mm ferrules were included, the results of this study might have been different. [37][38] Root canal instrumentations were simulated, and canal obturation procedures were accomplished, although canal obturation should have little or no effect on the strength of the roots. [2,27,[39][40] It has been suggested that a post should have the same modulus of elasticity as root dentin to distribute the applied forces evenly along the length of the post. [17,41] However, Creugers et al. [11] reviewed the related literature (studies that have been published in more than 20 years) and reported that survival rates have varied largely in endodontically treated teeth restored with different post-and-core systems. No consensus existed on which technique and materials are best suited for use; [9,11,42] some studies reported significantly higher mean failure loads for fiber posts, [9] or significantly higher mean failure loads for metal posts.
[ [43][44] In the current study, the greatest numbers of frac- consequently, less stress and more force distribution are exerted on the tooth. This phenomenon was also reported in previous investigations. [7,45] It has been suggested that the stiffness of the dowel in an endodontically treated tooth compromised by lost tooth structure can be a reinforcing medium. The results of Sidoli et al. [46] showed that the fracture strength of the teeth with carbon fiber dowels was lower than that for teeth with metal dowels. Since the dowels used in the present study had the same cross-sectional areas and shapes, the bending stiffness of the dowels would be directly proportional to the modulus of elasticity of each material. [12] Despite these measures, several factors limit the ameter and design of the post, canal shape and preparation, luting medium, method of cementation, [47] and location in the dental arch. [48] Since in this study all variables affecting post retention were the same, the retentive property of the two tested groups did not differ from each other significantly.
The data obtained from this study corresponds well to those found by other investigators in that the surface configuration of a dowel is the most important variable in retention. [14,49] It was shown that the retentive strength of the reinforced composite resin dowel-core systems may be increased significantly by roughening the dentinal walls or creating undercuts into the walls of the channel. Addition of pins may also promote the retention and resistance of this system. [8,[49][50] Retention and resistance to fracture are two important factors that must be achieved with post-and-core retained restorations. Nevertheless, retention often requires the removal of tooth structure, a procedure that may reduce the strength of the root. When placing a post, the dentist must evaluate each tooth individually to determine the best approach for obtaining the maximal fracture resistance. Because a single post system is unlikely to satisfy retentive requirements for all clinical situations, a variety of post systems are suggested to achieve the optimal balance between post retention and resistance to root fracture. This flexible approach should allow the dentist to successfully restore most endodontically treated teeth.

Conclusion
Teeth restored with Ni-Cr post and core system showed higher resistance to fracture compared with the group restored with NPG post and core system.NPG post and core system showed higher retention to tooth structure than Ni-Cr cast and core. Both post and core systems showed acceptable clinical behavior.